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488                        CTBWOMYS  AND   PETROMYS                      CHAP.
example of 120 mm. The tail too Is reduced, being in the same
example only 42 mm. in length. As in the last two genera the
large intestine is about one half of the length of the small
The " Tuco-tuco," genus Ctenomys, has also short ears and
tail. The claws of the fore-feet are longer than those of the
A related form is Aconaemys (better known as ScMzodon),
with similar external characters; it inhabits high localities on
the Andes.
Petromys is the only genus of the sub-family which is not
American in habitat. It is an African form and there is but one
species. Its anatomy conforms to that of the genera already
considered. The main difference in structure is shown by the
teeth. Their surface is uneven, and differs from that of other
Hystricomorphs " in that the enamel to the inside of each upper
jaw - tooth and outside on each lower jaw - tooth forms two
tubercles, to which correspond grooves in the reverse position of
the applied teeth."
Sub - Fam. 2. laoncherinae.  The genus Echinomys with
thirteen species belongs to the Neotropical region. The members of
the genus are entitled " Spiny Rats " since they have spines mixed
with the fur. The tail is long and the ears are very well developed.
Both feet are five-toed. The tail is scaly as well as haired.
Trichomys (also called JSTelomys) is very close to the above, and is
also from the same part of the world.
The genus Cannabateomys contains but one species, C.
amblyonyx, which was formerly included in the genus Dactylomys,
but has lately been separated by Dr. Jentink.1 The animal is
Brazilian and has a total length of 520 mm.,, of which 320 mm.
belong to the tail. It is a climbing rat, and in accordance with
that way of life has undergone some modifications. The fore-
feet are four-toed, the two middle toes being markedly longer
than the outer ones. The hind-feet are five-toed with the same
greater development of the two middle toes. The claws are
small and somewhat nail-like.
Dactylomys, also" Brazilian, and with but one species, JD.
dactyUnus, differs from the last in the fact that the molars are
simpler in form; they are divided into two lobes, each of which
1 Notes Leyd. Mus. 1891. u. 105.